Borehole nuclear magnetic resonance measurements provide different types of information about a reservoir. First, the measurements provide an indication of the amount of fluid in the formation. Second, the measurements present details about whether the fluid is bound by the formation rock or unbound and freely producible. Finally, the measurements can be used to identify the type of fluid--water, gas, or oil.
One approach to obtaining nuclear magnetic resonance measurements employs a locally generated static magnetic field, B.sub.0, which may be produced by one or more permanent magnets or electromagnets, and an oscillating magnetic field, B.sub.1, which may be produced by one or more RF antennas, to excite and detect nuclear magnetic resonance properties to determine porosity, free fluid ratio, and permeability of a formation. See U.S. Pat. No. 4,717,878 issued to Taicher et al. and U.S. Pat. No. 5,055,787 issued to Kleinberg et al. Nuclear spins align with the applied field B.sub.0 with a time constant of T.sub.1 generating a net nuclear magnetization. The angle between the nuclear magnetization and the applied field can be changed by applying an RF field, B.sub.1, perpendicular to the static field B.sub.0. The frequency of the RF field is equal to the Larmor frequency given by .omega..sub.0 =.gamma.B.sub.0 where .gamma. is the gyromagnetic ratio. After application of an RF pulse, the magnetization begins to precess around B.sub.0 and produces a detectable signal in the antenna.
Another approach to obtaining nuclear magnetic resonance measurements employs a locally generated static magnetic field, B.sub.0, which may be produced by one or more permanent magnets or electromagnets, and an azimuthally-oriented oscillating magnetic field, B.sub.1, which may be produced by one or more RF antenna segments that transmit and/or receive from different circumferential sectors of the logging device. See U.S. patent application Ser. Nos. 08/880,343 and 09/094,201 assigned to Schlumberger Technology Corporation. Typical long echo trains (.about.600 spin-echoes) are unobtainable with a rotating azimuthal antenna. Since the antenna is only properly positioned in the measurement direction during a short time, the signal decays faster due to rotation of the tool than it would due to formation properties alone.
U.S. Pat. No. 5,796,252 issued to Kleinberg et al. describes a nuclear magnetic logging device which includes permanent magnets, an RF antenna, and a coil for generating a magnetic field gradient. The technique described in the '252 patent utilizes pulsed magnetic field gradients to obtain information regarding diffusion properties of the formation fluids. If internal gradients are present in the formation, a pulse sequence is applied to reduce or substantially eliminate the effect of internal gradients in the formation. The '252 patent does not identify a method for using the coil to obtain an azimuthal NMR measurement.
U.S. Pat. No. 5,212,447 issued to Zvi Paltiel describes a nuclear magnetic logging device which includes permanent magnets and an RF antenna coil. The '447 patent requires a magnetic field gradient coil to determine a diffusion coefficient, i.e., the rate at which molecules of a material randomly travel within the bulk of the same material. The '447 patent employs the diffusion coefficient to determine at least one of the following petrophysical parameters: water/hydrocarbon discrimination, water and hydrocarbon saturation levels, permeability, pore size and pore size distribution, oil viscosity, a measure of the average increase in electrical resistance due to the formation tortuosity, and q-space imaging of the formation. The '447 patent does not identify a method for using the coil to obtain an azimuthal NMR measurement.
A primary object of this invention is to obtain an azimuthal NMR measurement. This measurement may be used to determine formation characteristics such as porosity, bound fluid volume, T.sub.2, T.sub.1, and permeability. Being able to measure the azimuthal variation of these characteristics is useful for interpreting heterogeneous formations and performing geologically based steering in deviated or horizontal boreholes.
Another object of the invention is to improve the vertical resolution of the tool using at least one gradient coil.